The present invention relates to a UV-crosslinkable composition comprising                a) unmodified, protonated silica nanoparticles;        b) urethane acrylate;        c) polar solvent; and        d) UV initiator system,wherein the amount by weight of unmodified, protonated silica nanoparticles exceeds the content of urethane acrylate and is at least 50.1 wt. %, based on the dry weight of the coating, to the use of the composition in the coating of substrates, and to substrates coated with such formulations.        
The principle of improving the properties of coatings by the integration of silica (silicon dioxide) has been known for a relatively long time. It is thereby possible, by adding silica particles, to improve coatings in respect of, for example, abrasion, scratch resistance, thermal deformability, reflection properties, gloss, antistatics, flammability, UV resistance, anti-fog properties, wettability with water and resistance to chemicals. If silica is used in the form of nanoparticles (particle sizes smaller than 100 nm), it should in principle be possible for these improvements in properties to be achieved while the transparency is at the same time retained, or is diminished only slightly. As is shown by the following literature citations, it has not hitherto been possible to combine all these property features, or even a relatively great combination of these features, in a surface-coating system. The latter is an object of the present application.
Accordingly, there has been no lack of attempts in the past to provide silica-containing coating compositions having further improved overall properties in respect of the above features.
DE 103 11 639 A1 describes moulded bodies provided with antistatic properties and a process for their production. In order to achieve the object, there are described in this connection surface-coating systems comprising acrylate-containing binders, alcoholic solvents, nano-scale electrically conductive metal oxides, nano-scale inert particles such as silicon dioxide and, optionally, further additives such as, for example, dispersing aids. The mean particle size of the inert nanoparticles used is from 2 nm to 100 nm, the particles being used in amounts of from 0.1 wt. % to 50 wt. %, based on the dry film.
JP 61-181809 discloses a UV-curable composition for coatings having good adhesion properties and high abrasion resistance, comprising α,β-unsaturated carboxylic acids and colloidal silicon dioxide particles, dispersed in water or low-valency alcohols.
JP 2005-179539 describes anti-fog coatings comprising from 20 wt. % to 99 wt. % of a mixture consisting of from 0 wt. % to 80 wt. % fine particles, for example silicon dioxide, and from 100 wt. % to 20 wt. % of a plastics material, as well as from 0.5 wt. % to 30 wt. % of a sulfosuccinate having two anionic substituents.
Surface-coating compositions based on polyfunctional acrylic acid esters for the production of coatings having high transparency, stability to weathering and scratch resistance are described in EP 0 050 996. In addition to the mentioned acrylic acid derivatives, the compositions comprise a polymerisation initiator as well as inorganic fillers such as, for example, silicon dioxide having mean particle diameters of from 1 nm to 1 μm and having a refractive index of from 1.40 to 1.60.
U.S. Pat. No. 4,499,217 describes anhydrous surface-coating compositions comprising colloidal silicon dioxide having mean particle diameters of from 10 μm to 50 μm and heat-curing compounds, for example acrylic compounds. The cured coatings exhibit good abrasion resistance as well as good adhesion to substrates.
JP 2001-019874 discloses compositions comprising (poly)ethylene glycol (poly)methyl methacrylate, acrylamides, photoinitiators, dispersing aids and silica for the production of coatings having good adhesion and increased scratch resistance.
WO 2006/049008 describes a hydrophilic coating based on silica particles which are suspended in a high-boiling solvent, such as N,N-dimethylacetamide; an alcoholic solution of a non-ionic surfactant (L-77) is added to the suspension, and tempering is then carried out for 10 minutes at 100° C. The coating yields a hydrophilic surface, it being possible to achieve wetting angles of 20° or less with water. This process is used in the coating of spectacle lenses in respect of anti-fog properties. However, these conditions are not suitable for the coating of plastics substrates owing to their sensitivity to the solvents used here.
A pourable formulation consisting of a mixture of an organic solution of polyvinyl-butyral and an alcoholic suspension of colloidal silica is described in U.S. Pat. No. 4,383,057. Based on dry weight, the composition can consist of from 20 wt. % to 95 wt. % polyvinylbutyral and from 80 wt. % to 5 wt. % silica. With regard to improving the stability values, such as scratch resistance, resistance to chemicals and inflammability, the polymer polyvinylbutyral is crosslinked, for which purpose there are used, for example, methylolmelanines modified with alkyl ethers. No further information is given regarding surface properties, such as hydrophilicity or water wetting angles. As compared with the present application, these are not UV-crosslinkable formulations.
If, as described in WO 2006/048277, surfaces with particularly high and dense silica structures are to be produced, the deposition of silica is frequently effected locally by flame hydrolysis from silica precursors, for example from hexamethyldisilazane or tetraethoxysilane. The hydrophobic nature of such coatings can be enhanced further by the integration of fluoroallylsilanes.
EP 0 337 695 discloses silicon dioxide dispersions for the abrasion-resistant coating of solid, in particular transparent, substrates. The dispersions comprise colloidal silicon dioxide having particle sizes smaller than 100 nm, preferably smaller than 75 nm, particularly preferably smaller than 50 nm, dispersed in a protically substituted ester or amide of an acrylic or methacrylic acid. From 0.1 to 2.5 parts by weight of silicon dioxide are employed per part by weight of unsaturated monomer that is used. The dispersions can be cured on suitable substrates by UV radiation after addition of a photoinitiator.
EP 0 505 737 describes UV-crosslinkable acrylate systems comprising methacrylate-functionalised colloidal silica nanoparticles. In addition to excellent weathering properties, the corresponding surface coatings exhibit good abrasion values, for example Taber haze of 6-8% after 500 cycles. The methacrylate-functionalised silica nanoparticles are products prepared from methacryloylpropyltrimethoxysilane and colloidal silica nanoparticles. Acrylate-modified silica nanoparticles have in the meantime also become available commercially, for example under the name “Nanocryl” from Nanoresins or “Highlink Nano” from Clariant.
These products, which are supplied as anti-scratch and anti-abrasion additives, are not very narrowly defined in terms of their properties owing to the complex chemistry.
It is an object of the present invention, therefore, to provide highly transparent hardcoat systems which have very good scratch resistance, abrasion values and weathering properties, while at the same time having low haze, and which adhere very well to various substrates. The haze, determined by the haze (H) values in accordance with ASTM 1003-00, should be less than 1% H, preferably less than 0.6% H. The abrasion values, determined in accordance with ASTM 1003-00, should be less than 12% H, preferably less than 8% H, particularly preferably less than 6% H, after 1000 abrasion cycles. The adhesion, determined by the cross-cut method, should exhibit ISO values of less than 2, preferably less than 1 and particularly preferably 0. These surface-coating properties should be retained in particular even after a boiling test of several hours, for example from 2 to 4 hours in boiling water, storage in water at elevated temperature for a prolonged period, as well as after weathering tests, for example artificial illumination/weathering in the Xenon-WOM (Weather-Ometer®) in accordance with ASTM G 26, G 151 or G 155. In addition, it should be possible to achieve as many as possible of the properties mentioned at the beginning, such as anti-fog, antistatic, hydrophilicity and resistance to chemicals, in the coatings according to the invention.
In particular for hardcoat systems having hydrophilic surface properties which exhibit the property profile according to the object, there continues to be an increased need, as compared with the prior art, for the provision of suitable formulations.
In addition, it should be possible to use these surfaces as a primer layer for further coatings, in particular from aqueous solutions which comprise, for example, cationic compounds.
It has been found, surprisingly, that most of the desired requirement profiles can be achieved to a high degree with the system according to the invention.